Ampero-Coulometry: A New Technique for Understanding Lithium-Sulfur Electrochemistry

Abstract

Despite limited commercial success, lithium sulfur technology (LST) is still far from replacing existing Li-ion technology. One of the main reasons hindering the success of LST is the limited understanding of lithium-sulfur chemistry during electrochemical charging and discharging. Dissolution of sulfur species in electrolyte solution further enhances the complexity of this system. Therefore, a comprehensive understanding of sulfur species and their kinetics during charge/discharge process is paramount for a high-performance lithium-sulfur battery. In this work, we used a chronoamperometric method to rapidly measure the rate capability of sulfur electrodes. The method allows us to interrogate rate performance of sulfur electrode from 0.1C up to 1000C in 26 mins of chronoamperometric measurement. Electrodes prepared with Super-P, carbon fiber and graphene platelets were compared to understand the relationship between electrode kinetics and geometrical porosity. Moreover, the electrical conductivity of sulfur electrodes with different thickness and material composition were compared to understand their dependence on C-rates. Finally, we developed a new technique 'Ampero-Coulometry' which utilizes the experimental results obtained from chronoamperometry and mathematically transform to reveal rate capability of carbon-sulfur electrodes at different state of charge/capacity (Figure 1a, b). This technique allowed us to track the overall rate of reactions inside Li-S cell over a complete state of discharge. For the first time, we were able to correlate Li+ diffusion coefficient with the chronopotentiometric discharge profile of a Li-S battery. As dissolution of sulfur species and their interplay inside a porous sulfur electrode has a significant role in limiting Li-S battery capacity, and we can now relate the known mechanism of polysulfide dissolution with the kinetics of a sulfur electrode and its performance.Figure 1: (a) Comparison of discharge curve and Ampero-Coulometry curve of lithium-sulfur battery (b) Comparison of discharge curve and differential rate curve of lithium-sulfur battery. Figure 1